Control of posture and stance in humans and animals is an active process involving central nervous system control. Responses to unexpected perturbations of posture are centrally generated patterns of muscle activity "triggered" by somatosensory cues signalling the event. We will continue research on the timing and organization of centrally generated postural responses to unexpected perturbations of the support surface. Particular emphasis will be on the rules for "mixing" of more than one muscle synergy and for modification of synergic organization as the direction of the perturbation is changed or as more than one perturbationis presented. Control of equilibrium depends on the integration of sensory inputs, primarily somatosensory, visual and vestibular, with motor outputs. The amplitude of the motor output is context dependent and can be manipulated experimentally by providing conflicting sensory information. The second portion of this proposal will use moving platform techniques to study the role of the vestibular system and neck proprioception in the modulation of centrally generated postural responses. Stabilization of visual and support surface cues will be used to study the relative contribution of these sensory modalities to posture and stance in chronic awake animals with vestibular deficits induced by hemi- or bilateral labyrinthectomy or by plugging semicircular canals. The inferior olivary system has been shown to respond to somatosensory events associated with unexpected perturbations of posture. The third goal of this proposal is a study designed to demonstrate the role of the climbing fiber system in the selection and performance of centrally generated postural responses using single unit recording techniques in chronic behaving animals during controlled perturbations of posture. Recordings will also be made from arrays of climbing fibers during perturbations to test the hypothesis that this system is a spatially coded event detector for unexpected tactile inputs.